An electric power steering apparatus, which urges a steering apparatus of an automobile or a vehicle with an assist load using the rotational force of a motor, auxiliarily applies the drive force of the motor to a steering shaft or a rack shaft through a transmission mechanism such as gears or belts via reduction gears. Such a conventional electric power steering apparatus executes a feedback control of a motor current for accurately generating an assist torque (a steering assist torque). The feedback control is to adjust a motor application voltage so as to reduce a difference between a current command value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of pulse width modulation (PWM) control.
A general structure of the electric power steering apparatus will be explained below with reference to FIG. 12. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of running wheels through reduction gears 3, universal joints 4a and 4b and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1. A motor 20 for assisting the steering force of the steering wheel 1 is connected to the shaft 2 through the reduction gears (speed reducer) 3. A control unit 30 for controlling the power steering apparatus is supplied with power from a battery 14 through an ignition key 11 and a relay 13. The control unit 30 calculates a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12. The control unit 30 then controls a current to be supplied to the motor 20 based on the calculated steering assist command value I.
The control unit 30 is mainly composed of a CPU. FIG. 13 shows general functions to be executed based on a program inside the CPU. For example, a phase compensator 31 does not show a phase compensator as independent hardware and shows a phase compensation function executed by the CPU.
Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then input thereby is phase-compensated by a phase compensator 31 for increasing the stability of the steering system. The phase-compensated steering torque TA is inputted to a steering assist command value calculator 32. A vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value calculator 32. The steering assist command value calculator 32 determines a steering assist command value I as a control target value of the current to be supplied to the motor 20, based on the inputted steering torque TA and the input vehicle speed V. The steering assist command value I is inputted to a subtracter 30A, and is also inputted to a differential compensator 34 of a feed forward system for increasing a response speed. A difference (I−i) calculated by the subtracter 30A is inputted to a proportional calculator 35, and is also inputted to an integration calculator 36 for improving the characteristic of a feedback system. The output from the proportional calculator 35 is inputted to an adder 30B as well as outputs from the differential compensator 34 and the integration calculator 36 are also inputted to the adder 30B. A current control value E which is a result of addition in the adder 30B is inputted to a motor drive circuit 37 as a motor drive signal. Further, a motor current is detected by a motor current detection circuit 38, and the detected motor current i is inputted to the subtracter 30A.
The electric power steering apparatus as described above includes the torque sensor 10 for detecting the steering torque transmitted by a steering shaft in order to apply a steering assist force according to the steering torque to the motor 20. Further, the electric power steering apparatus may include a rotation angle sensor (steering angle sensor) for detecting a steering angle (rotation angle) of the steering shaft to realize an accurate and excellent control, the rotation angle sensor being arranged independently as a sensor. That is, the rotation angle sensor can be retrofitted to a steering mechanism.
When the rotation angle sensor is mounted on the electric power steering apparatus, it is conventionally retrofitted to the steering mechanism from the outside. Accordingly, a size and shape of the rotation angle sensor are restricted by a structure, space and the like of the steering mechanism as well as the rotation angle sensor is disadvantageous in reduction in size and cost.
When the rotation angle sensor is not mounted, the rotation angle is estimated making use of a signal of other sensor, from which a problem arises in controlability.
Further, a brushless DC motor is often used as the motor of the electric power steering apparatus. In the brushless DC motor, since it is necessary to determine timing at which a field current is switched, the power steering apparatus includes a rotor position sensor for detecting a rotation angle position of a rotor. For example, rotation of a magnetized rotary member in a motor is detected as a pulse signal by a magnet detection device (Hall element or the like). Since the rotor position sensor outputs incremental information, it is possible to detect a relative rotational position of the motor from a start of operation thereof. However, it is impossible to detect an absolute value by the rotor position sensor.
More further, in the electric power steering apparatus using the brushless DC motor, an absolute rotation angle of the steering shaft just after the system starts operation cannot be discriminated. This is because that a driver does not always turn on an ignition key in a state that the steering wheel is located at a center as an operation start position. However, when a position of the steering shaft, which satisfies a predetermined condition after the steering apparatus operates, is identified as a neutral position of the steering shaft, an absolute angle of the steering shaft can be detected after the above position is identified. Contemplated as the predetermined condition are, for example, a case in which it can be recognized that a vehicle travels straight, a driver sets a steering torque to a predetermined value or less, and a predetermined period of time has passed, and the like. However, it is difficult to identify an absolute angle of the steering shaft just after traveling starts. Accordingly, a vehicle attitude controller, which prevents slip and the like based on the absolute rotation angle information of the steering shaft, and the like is disadvantageous in that they cannot exhibit a satisfactory function until an absolute angle can be recognized.
An object of the present invention, which was made in view of the circumstances described above, is to provide an electric power steering apparatus that includes a built-in type rotation angle sensor as a standard equipment without being mechanically restricted, rotation angle sensor being capable of detecting an angle with a high resolution by a simple structure. Another object of the present invention is to provide not only an incremental system rotation angle sensor but also a particularly useful absolute value system rotation angle sensor in a compact arrangement.